Terrestrial radiation comes largely from nuclear decay and the remnants of cosmic and solar radiation that has interacted with the atmosphere. This solar radiation is made up of light charged particles with energies of around 1 to 2 MeV.
Radiation in space is made up of particles having much higher energies. Space radiation can be divided into three types. Firstly, solar particles are ejected from the sun in solar flares. The magnitude of these varies according to the sun's 11-year solar magnetic cycle. The particles are electrons and protons with energies typically in the range of hundreds of MeV to low GeV. The protons cause the majority of the damage. Extreme events are known as solar proton events (SPE). Galactic cosmic rays (GCR) are a second type of space radiation. These consist of energetic particles from events in deep space. The particles are protons, helium nuclei, and small numbers of heavier nuclei such as carbon and iron. These particles often have very high energy, for example in the range 1 to 20 GeV. A third type of radiation is that trapped by the earth's magnetic field. This trapped radiation is formed by some of the flux of protons and electrons coming from the sun. They are concentrated into two radiation bands called the Van Allen belts. This radiation consists mostly of protons with energies up to 600 MeV.
In close earth orbit satellites benefit from the proximity of the earth and its magnetic field, but such satellites are outside the earth's atmosphere so they still receive significantly more radiation than on the surface of the earth. On earth space radiation produces an average dose of 0.4 mSv/year, whereas on the International Space Station this rises to 150 mSv/year. Deep space missions would subject humans to even greater doses, perhaps as high as 900 mSv/year.
Different orbits can have significantly different radiation environments. Satellites in a low earth orbit will have the lowest dose particularly those in an equatorial or low inclination orbit. Under normal circumstances radiation is unlikely to be life-limiting. Satellites in equatorial geostationary orbits are much further from the earth and will have a higher dose. But they still have a good chance of surviving for 10 years, which is typically the same as the lifetime of a communications satellite before it runs out of propellant.
Highly inclined or polar orbits that are used for earth observation satellites come into contact with the Van Allen radiation belts at the poles and at the South Atlantic Anomaly. The radiation damage on the electronics of these satellites is much higher and their life expectancy is correspondingly lower.
Communications satellites that are designed for use at high latitudes use a highly elliptical orbit called a Molniya orbit which takes them into regions of very high radiation and these satellites are unlikely to last longer than 7 years.
Lastly spacecraft that are designed to operate in deep space such as those used for monitoring solar weather or for exploration will encounter very high levels of radiation.
The amount of radiation that a satellite receives can vary dramatically during the sun's 11 year cycle, and can experience very high doses during extreme solar weather events. Some events irradiate to such an extent that the average power-draw can increase several percent in a few hours due to changes to semiconducting and dielectric properties.
Materials and electronics can be damaged by space radiation in several ways. Heavy ions, neutrons and protons can displace atoms in a semiconductor, introducing noise and error sources. The characteristics of capacitor dielectrics, metal resistor films, other passive electronic components and even wiring and cabling can be degraded by radiation over time.
It is also possible for high energy charged particles to alter the bits stored in computer memory. These are called single event upsets and can cause anything from a short-term denial of service to the loss of the satellite.
Most of the energy lost by an incoming particle in matter is through the interaction with electrons. So for space applications materials with the highest number of electrons per unit mass are best. However, very high energy irradiation can cause nuclear fragmentation in heavier elements such as aluminium, which can increase the absorbed dose for electronics or biological tissue behind the shield. In contrast, nuclear collisions become a significant cause of radiation particle energy loss when the materials contain a large fraction of light elements such as hydrogen. Unlike heavier elements, the nucleus of the hydrogen atom cannot fragment and instead slows the incoming ions through inelastic collisions. Thus a high hydrogen content shield material operates by absorbing or moderating the energy of the incoming particles.
For space applications weight is the critical factor and so the effectiveness of a radiation shielding material is measured by its ability to reduce the dose for the minimum weight, and the usual method for judging efficacy is a materials dose reduction versus its areal density (gm/cm2).
For this reason lightweight, hydrogenous materials such as polyethylene are preferred. Lead, on the other hand, is less efficient at absorbing energy per unit mass and is more suited to terrestrial situations where volume not mass is more important.
Polyethylene is 14.4% hydrogen by weight. Thus a key parameter in the choice of new materials is whether it has a hydrogen content greater than polyethylene.